This invention relates to an improved filler material for alkaline paper and an improved alkaline paper including the improved filler material. This invention also relates to a method of preparing alkaline paper and, more particularly, to a method for improving sizing of alkaline paper. 1.2 Background Information
Sizing agents are typically added to cellulose fibers during a paper making process to impart resistance to the paper to the penetration of liquid. Resistance to liquid penetration is necessary to prevent the paper from breaking down when the paper is passed through a size press starch solution prior to drying. Resistance to liquid is also necessary so that print quality can be maintained upon application of ink to the paper surface. Paper can contain sizing agents which are effective when the paper is prepared at neutral pH or alkaline pH ("alkaline paper") or at low pH ("acid paper"). Presently, sizing agents for alkaline paper that are typically used include alkyl ketene dimer and alkenyl succinic anhydride. These sizing agents covalently bond with the hydroxyl and carboxyl groups of cellulose fiber to impart resistance to liquid penetration to the paper sheet.
Fillers typically are added to cellulose fibers along with sizing agents during the paper making process. Fillers are added in order to improve, inter alia, brightness, opacity, show through and feel of the finally formed paper. Fillers used in alkaline paper making typically include finely divided inorganic particles such as clay, TiO.sub.2 and calcium carbonate. Currently, precipitated calcium carbonate ("PCC") is the preferred filler for producing alkaline paper of high brightness and high quality. However, such typically used fillers detrimentally affect the sizing of alkaline paper. These fillers have higher surface areas than the cellulose fibers and so adsorb the sizing agent, requiring additional sizing agent to be used during the paper making process.
It is known to modify the surface of filler used in alkaline paper making to enhance the performance of the filler such that the paper requires the addition of substantially less wet end sizing agent. For example, U.S. Pat. No. 5,147,507 to Gill describes calcium carbonate filler having cationic resin material coated thereon. It is also stated that the paper containing the modified filler has excellent opacity and tensile strength properties.
Coating the surface of calcium carbonate particles with a fatty acid has been described wherein calcium carbonate is used as a filler for acid paper. Since untreated calcium carbonate decomposes in acidic environments, calcium carbonate cannot be used as a filler in acid papermaking unless it is acid stabilized. Modifying the surface of calcium carbonate with a fatty acid was found to acid stabilize the calcium carbonate so that it can be used as a filler for acid paper. See, for example, U.S. Pat. No. 1,839,449 to Sutermeister et al.
Japanese Patent Application Announcement Sho 59-228098 of Asahi Glass K.K. discloses an inorganic particulate filler for use in manufacturing newsprint paper under acid conditions. The filler is calcium carbonate or calcium silicate coated with stearic acid, oxalic acid or salts thereof.
Fatty acids have also been used to coat calcium carbonate for purposes other than papermaking. For example, U.S. Pat. No. 4,753,679 to Damiano et al. discloses stearic acid coated precipitated calcium carbonate for use in cement. The Kirk-Othmer "Encyclopedia of Chemical Technology", Vol. 17, page 809 (John Wiley & Sons, New York 1982) discloses synthetic calcium carbonate surface-treated with stearic acid or rosin to improve rheological properties of the calcium carbonate when used as a pigment. U.S. Pat. No. 2,385,379 to Raften discloses mixing fatty acid with calcium carbonate to coat the calcium carbonate particles. In addition to acid stabilizing the calcium carbonate, this coating allegedly reduces the adhesive requirement and oil absorption of the calcium carbonate particles, thus having utility in the manufacture of paints, enamels and the like.
Presently, a need exists to improve the sizing of alkaline paper, such that adsorption of the sizing agent by the filler is reduced.